Integrated device with a structure for protection against high electric fields

Abstract: An integrated device presenting a structure for protection against electric fields. The protection structure is formed by a first region of conducting material (34) connected electrically to the gate/source region (6) of the device and at a first potential, and by a second region of conducting material (35) connected electrically to the drain region (15) of the device and at a second potential differing from the first. The first region of conducting material (34) is comb-shaped, and presents a first number of fingers (32) separated by a number of gaps; and the second region of conducting material (35) presents portions (33) extending at the aforementioned number of gaps and also forming a comb structure, so that the body of semiconductor material (1) of the device sees a protection region formed by a pair of interlocking comb structures and at an intermediate potential between the first and second potentials.

Description: The present invention relates to an integrated device with a structure for protection against high electric fields, "high" being intended to mean electric fields close to the critical field resulting in breakdown of the device. The state of the art is familiar with what are known as "field plate" structures which are formed over a semiconductor body for protecting the integrated components in the semiconductor body from external electric fields or those generated by the voltages applied to the components. Such structures are typically employed for high-voltage devices (ranging from a few tens to a few hundred volts) for ensuring correct operation of the device. To illustrate the problem involved, let us consider, for example, the case of an N-channel DMOS lateral transistor operating in RESURF (REduced SURface Field as described in "Thin layer high voltage devices (RESURF devices)", J. Appels et al., Philips J. RES., Vol 35, No. 1, pages 1-13, 1980) conditions and comprising normally bypassed source and body regions; a gate electrode; and a drain region formed by a lightly doped N - region known as a drift region, and by an N + type contact region. Such a transistor is shown in Figure 1, and comprises a wafer 1 including a P - type substrate 2, and an N - type epitaxial layer 3 forming a drift region 4. Epitaxial layer 3 houses a P type body region 5 in turn housing a source region 6 electrically connected to body region 5 by a contact 7 via a P + doped region 8. A polysilicon gate electrode 10 extends over surface 11 of wafer 1, and is insulated electrically from underlying body region 5 and epitaxial layer 3 by a thin oxide layer 13 and by part of a thick field oxide layer 14 extending over drift region 4 and interrupted at an N + type drain region 15. Over surface 11 of wafer 1 and gate electrode 10, there extends a PSG (Phosphorous Silicon Glass) isolating layer 16 interrupted at source contact 7 and gate and drain contacts 20 and 21, and the device is covered by a passivation layer 22. For the Figure 1 transistor to withstand a high inverse voltage V DSS (drain-source voltage with the source and gate bypassed), in addition to appropriately doping the various regions of wafer 1 and correctly sizing the oxide layers (gate oxide, PSG oxide), the extension of the metal gate and drain contacts 20 and 21 must also be sized correctly to prevent the formation of peak electric fields close to the surface (crowding of the equipotential lines shown by the thin lines in Figure 1) which would result in premature breakdown of the device. Moreover, for the structure to be reliable, i.e. to prevent its maximum breakdown voltage from varying with time, conduction or charge movement phenomena in dielectric layers 14, 16, 22 over drift region 4, and which would alter the electric field value of wafer 1, must be prevented. In other words, the reliability of the structure depends on the potential distribution between the drain and source/gate electrodes remaining unchanged with time. One conventional way of distributing voltage between the drain and source/gate electrodes, or more generally speaking between a high- and low-voltage electrode, is to use resistive materials such as SIPOS (Semi-Insulated POlycrystalline Silicon) as described for example in the article entitled "A 500V 1A 1-Chip Inverter IC with a New Electric Field Reduction Structure", by K. Endo, Y. Baba, Y. Udo, M. Yasui and Y. Sano, Proc. of the 6th Internat. Symposium on Power Semiconductor Device & IC's, Davos Switzerland, 31 May-2 June, 1994, pages 379-383, IEEE Cat. n. 94CH3377-9. Such a solution, however, fails to provide for ensuring the reliability of the structure, due to the electric characteristics of the material varying with temperature and the voltage applied. Other known solutions employ variously designed floating field plates, the performance of which, however, is less than satisfactory by virtue of employing floating electrodes which, not being referred to fixed potentials, are in themselves unstable. A high-voltage MOS transistor having field plates is known for instance from GB-A-2 077 493. The above considerations relative to a DMOS lateral transistor also apply to any other type of high-voltage structure, commencing with a straightforward diode comprising P + , N + regions embedded in an N - layer. It is an object of the present invention to provide a reliable structure for protection against the electric fields in high-voltage integrated devices, and which provides for ensuring high breakdown voltages and protection which is both stable with time and unaffected by temperature. According to the present invention, there is provided an integrated device featuring a structure for protection against electric fields, as claimed in Claim 1. Specific embodiments are disclosed in the dependent claims. In practice, according to the present invention, there is provided a field plate interdigitated between the positive electrode and the negative electrode, and which provides for distributing the equipotential lines in the wafer of semiconductor material in which the device components such as the above DMOS transistor are integrated, in such a manner as to prevent the formation of high electric fields in the wafer, and in particular protect the drift region from the effects of charge movements in the upper dielectric layers. A number of preferred, non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a cross section of a known DMOS transistor; Figure 2 shows a cross section in perspective of a DMOS transistor featuring a first embodiment of the structure according to the present invention; Figure 3 shows a top plan view of the layout of the Figure 2 protection structure; Figure 4 shows a variation of the Figure 3 layout; Figures 6-8 show the electric field of the Figure 4 variation at different sections of a silicon wafer and at different depths as shown in Figure 5; Figure 9 shows a top plan view of a further embodiment of the protection structure according to the present invention; Figure 10 shows a cross section, as in Figure 2, of a further embodiment of the protection structure according to the present invention; Figure 11 shows a cross section, as in Figure 2, of a further embodiment of the protection structure according to the present invention; Figure 12 shows a cross section, as in Figure 2, of yet a further embodiment of the protection structure according to the present invention. In Figure 2, the parts in common with Figure 1 (the DMOS transistor regions formed in and on wafer 1 of semiconductor material) are indicated using the same numbering system with no further description; and, for the sake of clarity, thin gate oxide layer 13 and passivation layer 22 are not shown in Figures 2 and 10-12. In the Figure 2 embodiment, a protection structure is formed over wafer 1 by gate and drain contacts 30 and 31, which respectively present a comb portion 34, 35 with respective teeth 32, 33, and the two comb portions are interlocked or interdigitated so that each tooth on "finger" 32, 33 of one comb portion 34, 35 is inserted, in noncontacting manner, between two adjacent fingers 33, 32 of the other comb portion 35, 34 (see also Figure 3). In the Figure 2 embodiment, fingers 32, 33 are all the same shape - rectangular - and the same size; and the passivation layer (not shown) is inserted in the gap between fingers 32 and 33. The protection structure according to the invention operates on the following principle. In the known transistor in Figure 1, in the absence of a protection structure and with the gate and drain contacts 20 and 21 facing each other along substantially parallel edges, the equipotential surfaces are distributed evenly between the two contacts in passivation layer 22 and wafer 1. With the Figure 2 comb structure, on the other hand, drift region 4 is entirely covered, except for the thin zig-zag region electrically separating the gate and drain contacts 30 and 31, so that each point is strongly affected by the combined action of the electric field generated by both contacts. In particular, by providing small fingers 32, 33 with respect to the thickness of oxide layers 14 and 16, and a small gap "d" between the fingers (width "l" of the fingers and gap "d" of the same order as the thickness of the oxide), the electric field "seen" inside wafer 1 at drift region 4 of length L D is equivalent to that which would be generated by a single electrode covering the whole of the drift region and at an intermediate potential with respect to those of contacts 30, 31. In the Figure 2 and 3 embodiment, wherein fingers 32, 33 are in the form of rectangles of the same width and length, each point of drift region 4 "sees" contacts 30 and 31 (i.e. fingers 32 and 33) at substantially the same distance and of the same area, thus achieving a field distribution equivalent to that generated by one electrode at a constant voltage equal to the mean potential of contacts 30, 31. The voltage in the drift region can be modified by altering the shape of the comb portion fingers. In the Figure 4 embodiment, the fingers 42, 43 are again all of the same shape, but in the form of a trapezium, in which case, drift region 4 presents an electric field equivalent to that generated by a variable voltage source increasing linearly from left to right (in Figure 4) as a function of the area ratio of fingers 42 and 43 at each surface section. In the Figure 5-8 embodiment, the fingers 52, 53 are triangular, and Figures 6-8 show the minimum and maximum equipotential lines V MAX , V min of the device at different depths of the dielectric layer formed jointly by field oxide layer 14 and PSG oxide layer 16. As can be seen, in the composite layer 14, 16, close to fingers 52, 53 of the contacts (Figure 6), the maximum and minimum voltage lines (equal to the potential of the contacts) present a pattern practically identical to the edge of the contacts; at an intermediate section (Figure 7), the indirect effect of the two contacts begins to be felt, so that the maximum and minimum voltage line pattern is more gradual; and, close to wafer 1 (Figure 8), the maximum and minimum potential line pattern is fairly straight with only small undulations. In the region between the maximum and minimum potential lines, potential distribution is linear. By appropriately selecting the width, shape and distance between the contacts and the distance between the contacts and wafer 1, the set potential may be made to follow on average the best potential distribution for the device in each case, thus eliminating the risk of premature breakdowns in wafer 1 (at junctions or induced by peaks) and so improving the stability of the electric parameters during the working life of the device. Figure 9 shows a further variation of the comb portion fingers of contacts 30 and 31, which presents curved fingers 62 formed by contact 30, and rectangular fingers 63 formed by contact 31. According to a further embodiment of the invention, the field electrodes may be formed by different layers deposited on the surface of wafer 1, and more specifically by appropriately shaping the polysilicon layer forming the gate electrode and/or the metal layer forming the contacts and connecting lines. In the Figure 10 and 11 embodiments, the comb structures are formed in different layers. More specifically, in Figure 10, the gate electrode 65 is so formed as to present a comb region 70 with fingers 71; the drain contact 72 also presents a comb region 73 with fingers 74 and located over but electrically separated from comb region 70 by PSG oxide region 16; and fingers 71 and 74 are mutually offset and so sized as to overlap slightly at the edges. Alternatively, fingers 71, 74 may be so sized as not to overlap, as in Figure 3, i.e. so that the width of each finger 71', 74' is less than the distance between two fingers of the other comb region 70', 73'. In both cases, the gate contact 75 is formed as in Figure 1, with no comb regions. In the Figure 11 embodiment, metal region 82 operating as the drain contact contacting drain region 15 comprises an unshaped portion 83 extending over drift region 4 and therefore covering the whole of comb region 70 of gate electrode 65 but isolated from it by dielectric layer 16. In this embodiment, fingers 71 of comb region 70 shield part of unshaped portion 83, so that only the unshielded parts of unshaped portion 83 are seen by drift region 4. Flat portion 83 therefore functions as though it, too, were comb-shaped, with fingers located precisely between fingers 71 of comb region 70 and of exactly the same size as the gap between fingers 71. The Figure 11 structure is functionally the same as that of Figure 10, but geometrically simpler. In the Figure 12 embodiment, both the comb regions are formed using the polysilicon layer forming gate electrode 65. More specifically, gate electrode 65 again presents comb region 70 with fingers 71; and the drain contact is formed in a number of parts, including a first polysilicon region 78, a second polysilicon region 79, and a metal region 90. The first and second polysilicon regions 78, 79 extend symmetrically with respect to drain region 15, and both present a portion contacting drain region 15, and a portion superimposed on field oxide layer 14. The portions of regions 78, 79 contacting drain region 15 are separated to permit metal region 90 to directly contact drain region 15; and the portion of each polysilicon region 78, 79 superimposed on the field oxide layer presents a comb region 92 with fingers 93. More specifically, comb region 92 of first polysilicon region 78 interlocks with comb region 70 of gate electrode 65; and comb region 92 of second polysilicon region 79 interlocks with another comb region 94 forming part of the integrated device in wafer 1, e.g. a further gate electrode. All the above embodiments operate as described with reference to Figure 2, so that the electric field in the underlying drift layer 4 is uniform, does not vary with time, and is such as to bring about no permanent temperature-induced change in the electric characteristics of the device. The advantages of the protective structure according to the present invention are as follows. Firstly, it provides for ensuring optimum, stable voltage distribution. In fact, by appropriately selecting the shape of the comb regions, it is possible to determine the potential seen by wafer 1; the potential is referred to constant predetermined voltages (source/gate and drain potentials); and the structure covers almost the whole of the drift region in the epitaxial layer, thus protecting the region most subject to breakdown. Secondly, the electric characteristics of the structure are stable, independent of the voltage applied, and result in no permanent temperature-induced alteration of the device by virtue of the structure comprising no resistive or capacitive dividers. Finally, any charge movements in the layers overlying the structure are fully shielded. Clearly, changes may be made to the structure as described and illustrated herein without, however, departing from the scope of the present invention as defined by the appended claims. In particular, and as already stated, the interlocking comb structures may be formed from the same layer of material shaped in the same step (polysilicon layer, metal layer), or from different layers of different materials deposited in different steps. Moreover, changes may be made to the shape of the fingers of the comb regions, depending on the potential distribution required; the fingers may be rectangular, triangular, polygonal (regular or irregular), in the form of a concave or convex arc, or of composite shape; spacing of the fingers may be constant or variable, depending on the potential distribution required; the fingers of the two comb structures (one high- and the other low-voltage) may be of the same or different shapes and sizes; and the comb regions may be actually formed as such, or, as shown in Figure 11, be shaped differently, providing they function as comb-shaped regions.

Claims: An integrated device comprising a body of semiconductor material (1) presenting a surface (11) and including at least a first device region (6, 10, 65) ; a second device region (15) ; and a structure for protection against electric fields and including potential distribution regions (34, 35; 70, 73; 82; 83; 92) extending over said surface (11) of said body of semiconductor material (1) between said first device region (6, 10, 65) and said second device region (15) ; said potential distribution regions comprising a first region of conducting material (34; 70) connected electrically to said first device region (6), and a second region of conducting material (35; 73; 82; 83; 92) connected electrically to said second device region (15); characterized in that said first region of conducting material (34; 70) is comb-shaped, and presents a first number of fingers (32; 71) separated by a number of gaps; and said second region of conducting material (35; 73; 82; 83) comprises portions extending at said number of gaps. A device as claimed in Claim 1, characterized in that said first and second regions of conducting material (34, 35; 70, 82; 92) are formed at a same level with respect to said body of semiconductor material (1) ; and said second region of conducting material (35; 92) is comb-shaped with a second number of fingers (33; 83) extending inside said number of gaps but electrically insulated from said first number of fingers (32; 71). A device as claimed in Claim 2, characterized in that said first and second regions of conducting material comprise a metal layer forming electrodes (30, 31) of said device. A device as claimed in Claim 2, characterized in that said first and second regions of conducting material comprise a polycrystalline silicon layer (65, 78) connected electrically to electrodes (75, 90) of said device. A device as claimed in Claim 1, characterized in that said first and second regions of conducting material (70, 73; 70, 83) are formed at two different levels with respect to said body of semiconductor material (1). A device as claimed in Claim 5, characterized in that said first region of conducting material (70) is formed from a polycrystalline silicon layer deposited on the surface (11) of said body of semiconductor material (1); and said second region of conducting material (73; 83) is formed from a metal layer deposited on said polycrystalline silicon layer (70) and electrically insulated from said polycrystalline silicon layer by a dielectric layer (16). A device as claimed in Claim 6, characterized in that said second region of conducting material (73, 73') is comb-shaped, and comprises a second number of fingers (74, 74') located over and at said gaps formed by said first number of fingers (71, 71'). A device as claimed in Claim 7, characterized in that the fingers (74) in said second number of fingers are larger than said gaps; and each finger (74) in said second number is superimposed partially on two adjacent fingers (71) in said first number. A device as claimed in Claim 7, characterized in that the fingers (74') in said second number of fingers are narrower than said gaps between the fingers (71') in said first number. A device as claimed in Claim 6, characterized in that said second region of conducting material comprises a plate region (83) over said first number of fingers (71) and said gaps. A device as claimed in any one of the foregoing Claims from 2 to 9, characterized in that said fingers (32, 33; 42, 43; 52, 53, 71, 74) in said first and second numbers of fingers have the same shape. A device as claimed in any one of the foregoing Claims from 2 to 9, characterized in that said fingers (62, 63) in said first and second numbers of fingers are of different shapes. A device as claimed in any one of the foregoing Claims from 2 to 12, characterized in that at least some of said fingers (32, 33) are rectangular. A device as claimed in any one of the foregoing Claims from 2 to 12, characterized in that at least some of said fingers (52, 53) are triangular. A device as claimed in any one of the foregoing Claims from 2 to 12, characterized in that at least some of said fingers (62) are curved. A device as claimed in any one of the foregoing Claims, and comprising a lateral DMOS transistor presenting a source region (6), a drain region (15), and a gate electrode (10; 65); characterized in that said first region of conducting material (34; 70) is connected electrically to said gate electrode (10, 65) ; and said second region of conducting material (35; 73; 82; 83; 92) is connected electrically to said drain region (15). A device as claimed in Claim 17, characterized in that said first region of conducting material (70) is formed by an extension of said gate electrode (65).

Inventors: CONTIERO CLAUDIO, DEPETRO RICCARDO, CONTIERO, CLAUDIO, DEPETRO, RICCARDO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H01L29
IPC codes: H01L29, H01L27, H01L21